1. Field of the Invention
This invention relates to video signal processing devices, and, more particularly, to video signal processing devices for processing signals by utilizing the vertical correlativity of video signals.
2. Description of the Prior Art
In the following a system is discussed for forming one frame by two fields taking as an example of an apparatus for obtaining one frame structure of video signals from one field of video signals by using line interpolation, as illustrated in FIG. 1. The main constituent parts of such apparatus comprise a 1/2H delay line 1 where H designates one horizontal scanning period, in the NTSC system, about 63.556 .mu.sec., a very short time delay line 2 for fine adjustment of the delay time on the order of 100 to several 100 nsec., a 1H delay line 3, two attenuators 4 and 5 for producing an output of amplitude 1/2 times lower than that of the input signal, a selector switch 6 and an adder 7.
Next, explanation is given to the operation of the apparatus of FIG. 1. As, for example, in the rotating magnetic sheet, one field of video signals was recorded in each round track, and is later continuously reproduced, a successive series of reproduced video signals for one field (hereinafter referred to as one "field signal") is assumed to arrive at an inlet 11 of FIG. 1. This field signal is supplied to a 1/2H delay line 1 where it is delayed by a time corresponding to the 1/2H, and of which the output is then applied to selector switch 6. Let us call the video output signal of the 1/2H delay line 1 1/2H signal. From a point of view of one frame of video signals (hereinafter referred to as one "frame signal") which will be finally obtained by the apparatus of FIG. 1, this 1/2H signal is a signal of, for example, the first field.
Also, the aforesaid field signal is applied to a delay line 2 and a 1H delay line 3 of which the delay times are respectively about 100 to several 100 nsec. and a time corresponding to 1H, and therefrom to respective 1/2 amplitude attenuators 4 and 5. The output signals of attenuators 4 and 5 (indicated at (a) and (b) in FIG. 1) are added by adder 7. Thus, adder 7 produces an output signal representative of the average of the preceding-by-1H video signal and the present video signal (indicated at (c) in FIG. 1) and which provides a signal of the second field for the frame signal. Here, the function of delay line 2 is to finely adjust the delay time error of 1H delay line 3 so that the difference in time between the signals (a) and (b) becomes exactly equal to 1H (63.556 .mu.sec.)
The thus-averaged signal becomes a so-called scanning line interpolation video signal. This signal and the aforesaid 1/2H signal are sent to selector switch 6 and are allowed to selectively appear at an outlet 12.
A vertical synchronization separator circuit 13, which is receptive of the video signals from inlet 11, produces vertical synchronizing signals which are then applied to a switching signal forming circuit 14. In synchronism with the vertical synchronizing signals, this circuit 14 produces switching signals controlling the operation of the selector switch 6 in such a manner that the 1/2H signal and the scanning line interpolation video signal at the output alternate in every vertical scanning period. Therefore, the output of selector switch 6 becomes a frame signal.
By such scanning line interpolation, the degree of vertical resolution of the field signal is increased from 262.5 lines (for note, in NTSC signal) to apparently 2 times as large, or 525 lines, improving the quality of the reproduced image.
In the apparatus of the character described above, however, it also results, even in the period including the vertical synchronizing signal, that the above-described treatment for obtaining the average signal by the line interpolation takes place. Therefore, an intermediate level is created for the vertical synchronizing component of the scanning line interpolation signal. FIG. 2 is a timing chart of wave forms appearing at various points (a) to (c) of FIG. 1, illustrating various fly-back time relationships to each other. In separating the vertical synchronizing signal out of the frame signal obtained by the apparatus of FIG. 1, therefore, the length of the vertical synchronizing signal as separated on the basis of the threshold level of the separator circuit is caused to alter from the proper 3H to 2H or 4H or the like. It also happens that the start point of the vertical synchronizing signal differs by 1H for the same reason. Upon consideration of the use of the separated vertical synchronizing signal for various desired purposes, therefore, problems have arisen.